Fetal Nasal Bone

In 1866, Langdon Down noted several features in individuals with trisomy 21 which included a small nose, flat facies, and poor skin elasticity. In 1997 Keeling, Hansen, and Kjaer reported an absent nasal bone in fetuses with trisomy 21. In November, 2001, Cicero et al reported in the Lancet the association between trisomy 21 and absence of the nasal bone in fetuses with trisomy 21 examined between 11 and 14 weeks of gestation. They found that the nasal bone was absent in 72.8% of fetuses with trisomy 21, 55% with trisomy 18, and 25% with Turner's syndrome. It was present in fetuses with trisomy 13, XXX, Klinefelter's syndrome, or triploidy. Using this marker, coupled with nuchal translucency and maternal age, a risk of 1 in 300 or higher will identify 92% of fetuses with trisomy 21 with a false-positive rate of 3% . However, before one can use the presence or absence of the nasal bone as a marker for trisomy 21, the ability to identify this structure accurately is necessary. This section will discuss normal anatomy, abnormal anatomy, and diagnostic errors that can occur when attempting to identify the presence or absence of the nasal bone during the first-trimester examination .

Identification of the Nasal Bone

The nasal bone actually consists of two bones, one on the right and one on the left. Traditional imaging using real-time B-mode ultrasound, can only identify one bone at a time. The image below this text illustrates this.

 This represents three imaging modalities that identify early fetal development. The fetus on the left is an actual photo at 12 weeks. The fetus in the middle is from an MRI study and the fetus on the right is an ultrasound. The nasal bone is identified in each image.

The visualization of the nasal bone was described by Cicero et. al. as follows:

"For examination of the fetal nose, a mid-sagittal view of the fetus was obtained, with the beam of the ultrasound transducer being parallel to the nasal bone. In this position, the skin of the nose produces an echogenic line, which can be misinterpreted as the nasal bone. To avoid this mistake, the ultrasound transducer was gently tilted from side to side to ensure that the nasal bone was seen separate from the nasal skin."

The Use of 2D Ultrasound to Evaluate the Nasal Bone

Evaluation of the nasal bone was first described using 2D ultrasound in which the nasal bone was identified in the sagittal plane, which only images one nasal bone at a time.  Since the nasal bone consists of two bones, Dr. DeVore prefers to identify both bones by directing the ultrasound beam in a transverse plane.  Using this approach both nasal bones can simultaneously be identified.  This is a similar approach described below using 3D ultrasound.

The Use of 3D Ultrasound to Evaluate the Nasal Bone

The most important part of obtaining the image of the nasal bone is proper alignment of the beam to image this structure. To assist in the examination of the nasal bone, 3D-multiplaner imaging is useful because it allows the examiner to align the sagittal image of the head and accurately identify the nasal bone. The following images demonstrates the proper sagittal image for identification of the nasal bone.

 This image demonstrates the plane of the falx, which is a midline structure of the brain. Lateral to the falx is the choroid plexus (CP) located on the right and left sides of the midline, which is present in the transverse and coronal images. The correct plane of the sagittal image of the head contains the falx, but not the choroid plexus. The tip of the nose is hyperechoic, but is not the location of the nasal bone.

 

 This image illustrates the planes of the coronal and transverse images using the sagittal image as the reference plane. The blue line in the sagittal plane corresponds to the transverse plane and the yellow line to the coronal plane. The nasal bone is identified by the purple arrows. Using this approach, the nasal bone can be accurately identified and confirmed to be present in all three planes

 

 This is a 3D holographic rendering of the facial bones. The green line indicates the plane of the rendering, which is located in front of the nasal bone. Examine the holographic image and identify the nasal bone.

 

 This compares two fetuses, in which the sagittal images were obtained in the midline in which the falx is present and not the choroid plexus. The nasal bone is present in the normal fetus and absent in the abnormal fetus with the increased nuchal translucency.

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